Start by poking holes in your prepared chocolate cake, as shown. I use the end of a wooden spoon.
In a microwave safe bowl add your marshmallow fluff and heavy cream. Place in the microwave for one minute. Stir vigorously until well combined and smooth.
Fill each hole with the mixture. Melt down your hot fudge and do the same. Place in the fridge for one hour.
Mix together your cool whip and hot cocoa mix. Ice your cake with the mixture. Top with marshmallows and grated chocolate. Place in the fridge for an additional hour.
Serve and enjoy. The only cake you will want this Christmas!